1. Field of the Invention
The present invention relates to apparatus and methods for thermally processing semiconductor substrates in semiconductor manufacturing, and in particular relates to support members (“chucks”) for supporting a substrate (semiconductor wafer) during laser thermal processing (LTP).
2. Description of the Prior Art
The fabrication of integrated circuits (ICs) involves subjecting a semiconductor substrate to numerous processes, such as photoresist coating, photolithographic exposure, photoresist development, etching, polishing, and in some cases heating or “thermal processing”. Thermal processing is used, for example, to activate dopants in doped regions (e.g., source and drain regions) of the substrate for certain types of ICs. Thermal processing includes various heating (and cooling) techniques, such as rapid thermal annealing (RTA) and laser thermal processing (LTP).
Various techniques and systems for performing LTP of semiconductor substrates (“wafers”) are known and are used in semiconductor device manufacturing. Example LTP systems and methods are described in U.S. Pat. No. 6,747,245 entitled “Laser Scanning Apparatus and Methods for Thermal Processing” (the '245 Patent), and in U.S. Pat. No. 6,366,308 B1, entitled “Laser Thermal Processing Apparatus and Method” (the '308 Patent), which patents are incorporated by reference herein.
LTP involves rapidly bringing the temperature of the wafer up to the annealing temperature and then rapidly back down to the starting (e.g., ambient or background) temperature in a single cycle. Given of the relatively large sizes of the typical wafers used in semiconductor manufacturing (e.g., 300 mm in diameter), the heat is more efficiently applied to only a small region of the wafer at a given time.
Using the '245 patent and the '308 patent as examples, a laser beam is formed into a narrow high-intensity image (e.g., a line image) that is scanned over the wafer surface, e.g., in a raster pattern. This process can involve a heat flux in excess of 1000 W/mm2 over the narrow image. The peak temperature TP reached by the wafer surface at the region being irradiated during LTP is relatively high (e.g., ˜1,300° C.).
The uniformity of the peak temperature TP determines the sheet resistance uniformity of activated doped regions formed therein, which in turn determines the performance of resulting semiconductor devices.
Attaining a uniform peak temperature TP over the wafer depends on the stability of the laser power and on the temperature uniformity of the wafer surface (referred to hereinbelow as the “background substrate temperature”). Maintaining a constant background temperature of the substrate, however, is problematic when the LTP process utilizes a spatially varying thermal load such as a scanned laser beam.
Accordingly, the art of LTP and related arts would benefit from apparatus and methods directed to maintaining the substrate being processed at a constant background temperature at the locations of the substrate not being directly subjected to the spatially varying thermal load.